Blood contains many kinds of proteins essential for supporting life. Of these proteins, albumin occupies the largest part in quantity and has many important functions.
Albumin in the blood makes fat soluble, while making a bond with substances harmful to living organism, thereby carrying these substances to the liver. Further, albumin plays an important role in maintaining the blood osmotic pressure. Albumin is also present in urine by a very small amount. The amount of albumin in urine reflects, for example, the level of filtering function of the glomeruli in the kidneys. An increase in the amount of urinary albumin indicates that the filtering function of the glomeruli in the kidneys may have deteriorated. If the increase of urinary albumin is not taken care of, one may suffer albuminuria, which is often symptomatic of kidney disease such as chronic nephritis. If such a disease is found, dialysis must be performed, and much worse, the patient may have kidney implanting surgery. In light of these, it is important to monitor the amount of albumin in urine so that an malfunction of the kidney can be found at an early stage.
As a technique which is usable for measurement of protein such as albumin, there is a simple measurement method called immunochromatography (see Patent Documents 1 and 2). The method is carried out by using an analytical element or a capillary strip containing a labeled antibody provided in a first region and a capture antibody (anti-albumin antibody) provided in a second region. More specifically, a liquid sample (urine for example) is supplied to the first region, whereupon capillary action of the strip causes the labeled antibody and the liquid sample to move to the second region. If the liquid sample contains the detection target antigen (albumin), the antigen combines with the labeled antibody as being moved to the second region. At the second region, the antigen-antibody combination is captured by the capture antibody. Then, based on the coloring of the antigen-antibody combination in the second region, it is possible to make a visual, qualitative determination on whether or not the urine contains a greater amount of protein than a predetermined level.
The immunochromatography is a qualitative method that enables visual determination of whether the result is negative or positive, so it is useful as a preliminary check for a functional impairment in the kidney. However, the immunochromatography is disadvantageous in that it cannot specify the level of the renal impairment quantitatively. In addition, the measurement takes a long time, often 10-15 minutes, until the liquid sample has completed its movement along the strip.
On the other hand, simple quantitative methods have been proposed for measuring the protein concentration. Well known examples are protein error method and dye binding method (see Patent Documents 3 and 4). The protein error method makes use of a phenomenon that a pH indicator called protein error indicator gives a color at a higher pH than the true pH, in proportion to the amount of protein contained in the sample. The dye binding method, on the other hand, makes use of a phenomenon that a maximum absorption wavelength of a dye shifts when the dye is combined with protein.
The protein error method and the dye binding method are highly convenient because they are simple methods to use. On the contrary, accurate measurement of protein is difficult in the protein error method and the dye binding method because the reagents used commonly in these measurement methods do not have high specificity to urinary albumin and therefore affected by a number of substances which are present in the urine. To date, various improvement efforts have been made for the protein error method and the dye binding method in order to increase measurement accuracy for example. Yet, there is still room for improvement in the protein error method and the dye binding method in the field of measurement accuracy for example.
There are methods which are less susceptible to influences from coexisting substances in urine. Examples include use of metal colloid (see Patent Document 5 for example), immunoturbidimetric method, and immunolatex agglutination method. In the colloidal metal method, a membrane is used to absorb protein. The membrane is then washed to remove coexisting substances. Then metal colloid is combined with the protein to determine quantity of the protein from the color of the metal colloid. The method is certainly less susceptible to influences from the coexisting substances, but is disadvantageous in that it requires complicated measurement operations such as the step of adsorbing protein in a membrane, the step of washing, and the step of allowing the metal colloid to combine with the protein. On the other hand, the immunoturbidimetric method and the immunolatex agglutination method are disadvantageous in terms of measurement cost since they require expensive regions.    Patent Document 1: JP-B-H07-13640    Patent Document 2: JP-A-H10-73592    Patent Document 3: JP-B-S57-51627    Patent Document 4: JP-A-S61-155757    Patent Document 5: JP-A-S63-127160